Polymer blending has great potential for the development of new materials with special properties for various applications. Successful polymer blending can also yield great ecological and economic advantages. The polymers used in the blends of the present invention are found in great quantities in the post-consumer waste stream. Thus, the "raw materials" of these blends are plentiful and inexpensive.
Typically, however, the simple mechanical mixing or blending of two or more polymers results in poor mechanical properties because most polymers are incompatible. Consequently, much effort and capital has been expended in devising methods for compatibilizing polymers and formulating polymer blends.
Polyethylene and polystyrene are two of the most common, and therefore commercially significant, polymers. Consequently, it has long been the object of research and development efforts to devise reliable and economical means for generating stable and compatible PS/PE blends with improved mechanical properties. See, e.g., W. M. Barentsen and D. Heiken, Polymer 14, 579 (1973); C. E. Locke and D. R. Paul, J. Appl. Polym. Sci. 13, 308, (1983); R. Fayt, R. Jerome and Ph. Teyssie, J. Polym. Sci., Polym. Lett. Edn. 19, 79 (1981); R. Fayt, R. Jerome and Ph. Teyssie, J. Polym. Sci., Polym. Phys. Edn. 20, 2209 (1982); R. Fayt, R. Jerome and Ph. Teyssie, Makromol. Chem. 187, 837 (1986); R. Fayt, R. Jerome and Ph. Teyssie, J. Polym. Sci., Polym. Lett. Edn. 24, 25 (1986); R. Fayt, R. Jerome and Ph. Teyssie, J. Polym. Sci., Polym. Phys. Edn. 27, 775 (1989); R,. Fayt, R. Jerome and Ph. Teyssie, Polym, Eng. Sci. 30, 937 (1990); R. Fayt, R. Jerome and Ph. Teyssie, J. Polym. Sci., Polym. Phys. Edn. 33, 801 (1995); C. R. Lindsay, D. R. Paul and J. W. Barlow, J. Appl. Polym. Sci. 26, 1 (1981); M. C. Schwarz, H. Keskkel, J. E. Barlow and D. R. Paul, J. Appl. Polym. Sci. 35, 653 (1988); M. C. Schwarz, J. W. Barlow and D. R. Paul, J. Appl. Polym. Sci. 35, 403 (1988); W. E. Baker and M. Saleem, Polymer 28, 2057 (1987); M. Saleem and W. E. Baker, J. Appl. Polym. Sci. 39, 655 (1990); J. W. Teh and A. Rudin, Polym. Eng. Sci. 31, 1033 (1991); J. W. Teh and A. Rudin, Polym. Eng. Sci. 32, 1678 (1992); P. V. Ballegooie and A. Rudin, Polym. Eng. Sci. 28, 1434 (1988) (all incorporated herein by reference).
One of the most frequently used methods to compatibilize PS/PE blends is the incorporation of a block copolymer such as a block copolymer of polystyrene and polyethylene (PS-b-PE) or a graft copolymer such as polystyrene-g-polyethylene (PS-g-PE) as a compatibilizer. The beneficial effects of the compatibilizer on the morphological and mechanical behavior of the blends is well demonstrated. A small amount of the compatibilizer reduces the phase size, stabilizes the phase morphology against coalescence, and increases the interfacial adhesion. The effects of the molecular weight, the composition, and the molecular structure of the copolymer on the blend miscibility or compatibilization have also been extensively studied. However, the costs of synthesizing these tailor-made compatibilizers make this route commercially unattractive.
Another popular approach has been to introduce reactive groups onto each of the two polymers to be blended. These functionalized polymers can then form the required compatibilizer during a subsequent reactive extrusion. However, this approach also requires separate processes to produce the functionalized polymers.
An alternative to that approach that has been investigated is the use of peroxide-initiated functionalization leading to grafting or cross-linking reactions. But that alternative proved to have limited success because of difficulties in achieving optimum conditions with minimum levels of intra-species crosslinking of PS and PE, and because of chain degradation.
U.S. Pat. No. 3,445,543 (incorporated herein by reference) describes high impact polymer compositions of monovinyl substituted aromatics. The '543 patent describes a blend of: a homopolymer of a monovinyl substituted aromatic compound such as styrene; a block copolymer such as those formed of a minor amount of a conjugated diene (e.g., butadiene) and a major amount of a monovinyl substituted aromatic compound (e.g., styrene); and a rubbery ethylene-1-olefin polymer (e.g., ethylene-propylene copolymer). The '543 patent teaches that the homopolymer, block copolymer, rubbery ethylene-1-olefin polymers, and cross-linking agent can be mixed together in any order and heated to effect cross-linking. However, as demonstrated below, the mechanical properties of blends resulting from the random combination of such components are variable and suboptimal. More particularly, those blends are acknowledged as having improved aging properties, but not improved impact strength, tensile strength or elongation. (See Table II)
U.S. Pat. No. 4,469,847 (incorporated herein by reference) describes graft styrene copolymers from a two-stage polymerization of styrene in the presence of an ethylene/propylene/polyene monomer (EPDM) elastomer and a styrene block copolymer. The two-stage process involves prepolymerizing styrene in the presence of an elastomeric EPDM terpolymer and styrene block copolymer, until polymerization is about 20% complete; and effecting a suspension polymerization of the resultant mixture in the presence of water such that the weight ratio of organic phase:water phase is from 0.8:1 to 1.3:1. Such two-stage polymerization processes are inefficient due to the necessity for monitoring the reactants and properly timing and implementing the various stages of the process.
Despite the foregoing, the art lacks a simple, expedient, and reliable process for blending otherwise incompatible polymers such as polyethylene and polystyrene to produce materials having desirable mechanical properties.